Make JuMP tests callable from an external package

[blegat]

The tests that are run for JuMP.Model and JuMPExtension.MyModel should also work for StructJuMP.StructuredModel. Currently, if StructJuMP wanted to run these tests, it would have to include, e.g. constraint.jl and then call constraints_tests(StructuredModel, ...) but including constraint.jl also run the tests for JuMP.Model and JuMPExtension.MyModel. It would be nice to define the tests and run the tests in separate files so that StructJuMP can define them without running them on JuMP.Model and JuMPExtension.MyModel.